How to restrict to cycles use only 90% of gpu?
From the Blende manual on GPU Rendering


  Why is Blender unresponsive during rendering? 
  
  While a graphics card is
  rendering, it can not redraw the user interface, which makes Blender
  unresponsive. We attempt to avoid this problem by giving back control
  over the GPU as often as possible, but a completely smooth interaction
  can’t be guaranteed, especially on heavy scenes. This is a limitation
  of graphics cards for which no true solution exists, though we might
  be able to improve this somewhat in the future.
  
  If possible, it is best to install more than one GPU, using one for display and the other(s) for rendering.